Photopolymers are organic polymers that have the unique property that when exposed to light of an appropriate wavelength, they are chemically changed in their solubility in certain solvents. Photopolymers find use in a number of applications, such as adhesives and photoresists. Photoresists are widely used in the electronic industry in the manufacture of semiconductors and printed circuit boards. Two types of photoresists are available: negative acting and positive acting. Negative acting photoresists are initially soluble in a developing solution, but after light exposure, they become polymerized and insoluble in the developer. Positive photoresists work in exactly the opposite fashion. Exposure to light makes the polymer more soluble in the developing solution. Exposure of photoresists to the light is typically made through a film pattern, or it can be done using a laser or other type of light beam that is rastered across an area. The photoresist pattern that remains after development is insoluble and chemically resistant to other solutions, such as the cleaning, plating, and etching solutions commonly used in producing printed circuit boards.
Since photoresists have typically been used as masking agents for etching and other types of solutions, the developing process for photoresists has been an "all or none" situation. That is, process conditions have been optimized in order to completely remove the photoresist from the desired area during a developing cycle. This provides a hole or opening in the photoresist so that the underlying substrate, if any, is exposed. While this is a desirable situation for the production of semiconductors and printed circuit boards, it provides a severe limitation on photoresists that allows them to only be used in these types of applications. It would be highly desirable if a three-dimensional structure could be fabricated using a photoresist, thereby opening the field to many more applications and uses for photopolymers.